1. Field of the Invention
The present invention relates to a picture decoder, and more particularly to an improvement in picture quality degraded by a data error or a data loss caused during picture data transmission.
2. Description of the Prior Art
Recently, standardization of picture coding methods has been under way, with the implementation of moving-picture transmission systems in mind. Those systems include a videotelephone, videoconferencing, and video-on-demand (VOD). ITU-T Recommendation H.263 and MPEG (Moving Picture Experts Group) are well known as international standardization standards.
For example, the coding method adopted by the ITU-T Recommendation H.263, shown in FIG. 2, performs intra-frame coding on time-sequenced frames (I frames a and i) at regular intervals and, at other times, performs inter-frame coding for each of P frames (inter-frame coded frames, b-h, j-k) with reference to the immediately preceding frame to remove temporal redundancy. In the following discussion, a frame coded in the intra-frame coding method is referred to as an I frame, while a frame coded in the inter-frame coding method is referred to as a P frame.
This technology is described in "Standardization of Multimedia Coding" by Hiroshi Yasuda, pp. 84-97, Maruzen, (1991).
The method proposed by Recommendation H.263, which performs the inter-frame coding of each frame by referencing the immediately preceding frame, requires that all the frames be transmitted in the correct sequence. For a telephone line or an ISDN line over which data is transmitted after a connection with a partner is established, data reaches the partner without loss and in the correct sequence.
However, for an Ethernet LAN or an ATM network in which data is divided into small units (called packets or cells) before it is transmitted, there is a possibility that packets are lost or transmitted in an incorrect sequence.
In general, networks employ a protocol (for example, TCP: Transmission Control Protocol) in which the transmitting device sends packets with attached serial numbers, and the receiving device rearranges the packets in the correct sequence, confirms their arrival, and sends requests for the retransmission of non-arriving packets back to the transmitting device in order to deal with these problems and increase network reliability.
However, when network operation is unstable and packets are dropped frequently, retransmission under this type of protocol can cause large cumulative delays to build up, which is inappropriate for the real-time transmission of moving pictures. In some cases, it is preferable to display new data, even if that means skipping a frame, rather than retransmitting old data, especially when new data can be displayed immediately.
Broadcasting and multicasting are schemes which send data to a plurality of sites at a time. However, when packet dropout occurs during transmission of a packet to one of the sites, the above protocol requires that the same packet be sent even to those sites which have received the packet successfully, significantly increasing the network load.
Broadcasting and multicasting are therefore performed using a protocol that does not re-transmit a packet, such as the User Datagram Protocol (UDP); as a result, the probability of packet dropout increases.
In wireless networks, the data error rate or data drop-out rate is high not only when data is sent in packets but when a line connection is established before data is sent. In addition, when the errors exceed the error-correcting capability of the receiving device, a sequence of data items are sometimes discarded to receive some other part of data successfully. Data dropouts in wireless networks therefore tend to be larger than in wireline networks.
Another problem is that the processing speed of the sending device is not always equal to that of the receiving device. For example, decoding all the frames on a slower receiving device would put much of the frame data in the wait state, causing long delays. This requires the receiving device to intentionally skip frames. However, when there is no decoding data for a frame preceding the current frame, as in the inter-frame coding method according to the prior art, the current frame cannot be decoded and therefore frames cannot be skipped as intended.
FIG. 3 shows an example of frame dropout that may occur during transmission of frames. When frame e is dropped out or cannot be decoded because of slow processing, the P frames (f, g, h) cannot be decoded until the next I frame, i, is received.
Thus, in order to send all the frames successfully in a network in which frame dropout or frame skipping occurs frequently, intra-frame coding for all the frames is more efficient. However, the problems with this method, which does not use inter-frame coding, are that there is temporal redundancy and that data transmission efficiency is degraded.
Thus, to successfully transmit moving-picture data over a network in which a data dropout occurs frequently, there has been a long felt need for a picture coding system which ensures high coding efficiency and quick recovery from degraded picture quality.